1. Field of the Invention
This invention relates to a strain detector for detecting a strain of a driven shaft such as, for example, a rotary shaft and also to a process of producing such strain or detector.
2. Description of the Prior Art
Generally, where a driven member to which an external force is applied is made of a magnetic material, a strain is produced in the driven member by the external force, and the magnetic permeability of the driven member varies in response to such strain. Accordingly, a degree of such strain can be detected as a variation in magnetic permeability by flowing magnetic flux through the driven member. Thus, a strain of such a driven shaft made of a soft magnetic material having a high magnetic permeability is conventionally detected in such a manner that a magnetic shielding layer of a non-magnetic material having a high electric conductivity is formed at a portion of a surface of the driven shaft, while and a magnetic layer is formed at the remaining portion of the surface of the driven shaft at which the magnetic shielding layer is not formed. A variation in the magnetic permeability of the magnetic layer is detected to detect such strain.
In the conventional strain detector as described above, since the magnetic layer and the magnetic shielding layer are exposed outside, the change of the strain detector with the passage of time is so great that the sensitivity of the strain detector at an initial stage cannot be maintained long. Consequently, the strain detector is low in reliability.
An exemplary conventional strain detector is disclosed, for example, in Japanese Patent Laid-Open No. 57-211030. Such a strain detector is shown in FIG. 13. Referring to FIG. 13, a driven shaft 1 in the form of a rotary shaft is supported for rotation around a center axis thereof by suitable bearing means not shown. A pair of magnetic layers 2 made of a soft magnetic material having a high permeability and suitable magnetostriction are fixedly mounted on an outer periphery of the driven shaft 1 in a spaced relationship from each other in an axial direction of the driven shaft 1. Each of the magnetic layers 2 is composed of a plurality of parallel layer stripes which extend at angles of +45 degrees and -45 degrees with respect to the center axis. The permeability of each of the magnetic layers 2 thus varies in response to an amount of a strain which is caused by a torque applied to the driven shaft 1. A pair of detecting coils 3 are disposed in a spaced relationship around the magnetic layers 2 on the driven shaft 1 for detecting a variation in magnetic permeability of the magnetic layers 2.
With the strain detector of the construction above construction, if an external torque or force is applied to the driven shaft 1, then a tensile force is produced on either one of the magnetic layers 2 while a compression force is produced on the other magnetic layer 2, thereby causing the magnetic layers 2 to be distorted. The magnetostriction of the magnetic layers 2 allows the orientation of the magnetization within each domain to be altered by such strain, and then the permeability of the magnetic layers 2 is varied. In this instance, the permeability is varied in the opposite direction whether the strain is caused by a tensile force or a compression force, and consequently, the permeabilities of the magnetic layers 2 are varied in opposite directions to each other. The detecting coils 3 detect variations in permeability of the corresponding magnetic layers 2 as variations in magnetic impedance and thus detect an amount of torque applied to the driven shaft 1 and an amount of a strain of the driven shaft 1 produced by such torque.
In the conventional strain detector shown in FIG. 13, however, since the driven shaft 1 and the magnetic layers 2 fixedly applied to the surface of the driven shaft 1 are formed as separate members from each other, a thermal stress is produced in each of the magnetic layers 2 due to a difference in coefficient of thermal expansion between the driven shaft 1 and the magnetic layers 2. Such thermal stress is overlapped with a stress caused by a strain to make an error in measurement of an amount of the strain of the driven shaft 1. Thus, in order to eliminate such error, it has been proposed to form a driven member from a soft magnetic material having a high permeability and selectively form a magnetic shielding layer of a non-magnetic or diamagnetic material having a high electric conductivity at a portion of a surface of the driven member while magnetic layers each composed of a plurality of parallel layer stripes are formed at the other portion of the surface of the driven member at which the magnetic shielding layer is not formed. In this instance, since the driven member and the magnetic layers are formed in an integrated relationship, the problem of a thermal stress described above is eliminated.
With the strain detector described above, however, since the magnetic shielding layer only covers over the surface of the driven member, magnetic fluxes do not readily penetrate into the individual magnetic layer stripes constituting the magnetic layers, and consequently, the magnetic shielding effect of the magnetic shielding layer is low. As a result, the variation in magnetic permeability by a strain of a magnetostrictive layer is small and the output sensitivity of the strain detector is deteriorated.
Further, such conventional strain detector as described above normally adopts vapor deposition, plating, ion plating, or the like as a method of forming a magnetic shielding layer thereof. Any of such methods, however, requires a long period of time to obtain a sufficient film thickness to shield the magnetism and is low in operability. Particularly, conventional electroplating of copper requires about 6 hours to obtain a desired film thickness. Besides, the magnetic shielding layer must necessarily be formed only at a required portion of a driven member, which also deteriorates the operability.